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Summary Analytical NMR Spectroscopy
Summary study book NMR Spectroscopy of Harald Günther, Harald G?nther (Chapter 1-12) - ISBN: 9783527330003, Edition: 3rd Edition, Year of publication: - (Complete summary)
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Summary Analytical NMR SpectroscopyChapter 1:
- NMR measures the absorption of electromagnetic radiation in the radio-frequency region →
400 (9.4 T) – 900 (18.8 T) MHz)
- Nuclei are involved in absorption process
- Sample need to be place in strong magnetic field to cause different energy states
- Probe the composition, structure, dynamics and function of the complete range of chemical
entities
- NMR is routinely and widely used to rapidly elucidate chemical structure
- Only odd numbers of protons are NMR active: H1, C13
Applications: structural (chemical) elucidation, study of dynamic processes, reaction kinetics and
study of equilibrium, structural (3D) studies, metabolomics, drug design and medicine MRI
Energy level of the molecule splits → low and high energy range → give resonance signal
- Interaction of nuclear magnetic moment with external magnetic field (B0) leads to nuclear
energy diagram
- Through radiofrequency (RF) transmitter, transitions between these states can be stimulated
→ absorption of energy is detection in RF receiver and recorded as spectral line →
resonance signal
Information in spectrum:
- Position: chemical shift
- Splitting: coupling constant → neighboring nuclei
- Intensity: integral → nuclear count
- Shape: line width → molecular motion (alcohols often wider due to hydrogen bonding)
Example: ethyl formate
- Protons exist in different chemical environment
(CH3, CH2 and OH) → give rise to different signals
- Resonance signals are separated by chemical shift
- Splitting (singlet, triplet, quartet) is the result of
spin-spin coupling → coupling between nuclei on
adjacent atoms → adjacent protons + 1
Temperature dependence:
- NMR spectra are temperature dependent and sensitive to dynamic processes
- The cause of this different behavior at two temperatures is the high barrier to rotation
-
- High temperature: very fast conversion → only one peak
- Low temperature: double bond between C-N gives rise to 2 peaks
Important developments:
- Introduction of cryomagnets with high magnetic fields → provided by superconducting coil
- Improved sensitivity → lower spin state becomes more highly populated (higher N)
- Replacement of continuous wave (CW) methods by pulse Fourier transform method
- Introduction of two-dimensional NMR
,Chapter 2:
Nuclear spin:
- Nuclear of an atom is comprised of nucleons (neutron and proton)
- Nucleons have a spin similar to angular momentum
- S = spin quantum number
- If #neutron and #protons are both even → S = 0
- If #neutrons + #protons is odd → S = 1/2, 3/2, 5/2
- If #neutrons and #protons are both odd → S = 1, 2, 3
- So odd neutrons/protons give spin of ½
- 𝑆 = √𝑠(𝑠 + 1)ℏ
- Number of states: 2S + 1
- Nuclear spins align with the magnetic field (low-energy)
- Magnetic field always along z-axis (figure)
Difference in energy between two states:
E = h B
- B0: external magnetic field
- : gyromagnetic ratio = 26753 s-1 gauss-1
- h: Plank’s constant
- Energy difference is proportional to magnetic field strength
Larmor equation: frequency of absorption: v = B
ΔE γh𝐵0
𝑁
Boltzmann relation: 𝑁𝛼 = 𝑒 𝑘𝑇 = 𝑒 2𝜋𝑘𝑇
𝛽
- Distribution of protons between ground and excited state
- Standard: E for 1H at 400 MHz (B0 = 9.39 T) is 6 x 10-5 kcal/mol
Stronger magnetic fields → increase population ratio → increase sensitivity
Resonance phenomenon:
- Left-hand-rule: thumb points along B1, the bend fingers show the sense of rotation
- Classical view:
- Nuclei either align with or against external magnetic field along the z-axis
- Since more nuclei align with field, net magnetization exists parallel to external
magnetic field
- Quantum description: nuclei either populate low energy or high energy (alpha or beta)
- Resonance can be realized in two ways:
- Varying the frequency at constant field
- Varying the magnetic field strength while keeping frequency constant
NMR signal generation:
- RF pulse, a strong RF field of short duration causes nuclear excitation
- Pulse excitation always occurs at constant field
- M rotates from the z-axis in the direction of the y-axis
- RF radiation end and only the magnetic field acts upon M, which starts a precession around
the z-axis with the Larmor frequency characteristic of the particular nucleus
- The time signal induced in the receiver coil through this motion of the x,y component of M,
S(t), the so-called free induction decay (FID), fades away through relaxation
- FT the frequency signal to gain NMR spectra
Relaxation: process for energy loss experienced by nuclei in the excited state
, Probe(-head): sample holder, air turbine, and transmitter and receiver coils
Magnetic shielding:
- Some nuclei are more/less (de-)shielded than others
- Surrounded by more electrons → more shielded
- Circulation electrons create an induced magnetic field that opposes the external magnetic
field
Chapter 3:
- Random orientation without magnetic field
- If magnetic field is applied, nuclei will be in alpha or beta state (alpha more populated)
- RF: excitation from alpha to beta (beta more populated)
- Time signal (FID) → Fourier transform to frequency signal
- Magnetic field increases from left to right, frequency increases from right to left
- Chemical shift is caused by electron in C-H bond
- External magnetic field induces circulations in electron cloud surrounding nuclear such that
magnetic moment (opposed to B0) is produced
- Local field at nucleus is smaller than applied field Blocal = B0(1-)
- : shielding constant → proportional to electron density → more electrons surrounding the
nuclei, the more shielded it is (less prone to ‘see’ magnetic field)
- Shielding effects at nucleus A caused by the secondary magnetic field arising from induced
electronic currents at nucleus B → deshielding
- = dialocal + paralocal + ’
- ’: neighboring contributions
- Paramagnetic effects arise only for nuclei where energetically low-lying atomic
orbitals are available → so not for protons
Chemical shifts:
- Position of resonance signal is measured relative to reference compound → often TMS
- Low → highly shielded
- Only relative number of protons can be determined by integration
𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 𝑜𝑓 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑜𝑛 𝑏𝑦 𝑠𝑎𝑚𝑝𝑙𝑒 𝑛𝑢𝑐𝑙𝑒𝑢𝑠 𝑖𝑛 𝐻𝑧−𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 𝑎𝑑𝑠.𝑟𝑎𝑑.𝑏𝑦 𝑇𝑀𝑆 𝑖𝑛 𝐻𝑍
- = 𝑆𝑝𝑒𝑐𝑡𝑟𝑜𝑚𝑒𝑡𝑒𝑟 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 𝑖𝑛 𝑀𝐻𝑧
- So frequency data are field dependent
- More deshielded (higher ) when next to electron withdrawing group (e.g. oxygen group)
- Shielding decreases: CH3 > CH2 > CH → CH3 has lowest
𝑁𝐵 𝐴𝐴 𝑀𝐴
Integration: 𝑚𝐴 = 𝑚𝐵 ∙ ∙ ∙
𝑁𝐴 𝐴𝐵 𝑀𝐵
- m: mass N: number of protons A: area under signal M: mol weight
Spin-spin coupling:
- Multiplicity occurs due to coupling of nuclei on adjacent groups → magnetic interaction
between protons that is not transmitted through space but by bonding electrons
- Multiplicity = neighbors + 1 = n + 1
- Low-energy state corresponds to antiparallel arrangement of nuclear and electron magnetic
moments ( = + ½ = - ½)
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